The present invention relates to an apparatus for the dry storage of heat-emitting materials, more particularly usable for storing radioactive materials.
It applies to all types of irradiated fuels or to certain categories of nuclear waste. It more particularly relates to fuels from experimental nuclear reactors produced in limited numbers or having a low content of recoverable fissile materials, non-standard fuels, various waste materials, such as the residues of cans or fission products for which reprocessing or final storage are not yet possible.
The solution used for the control of these objects is long term intermediate storage (several dozen years), after which a final processing will take place.
For storing heat-emitting irradiated materials, three main conditions must be fulfilled:
the confinement of radioactive materials preventing the contamination of the environment;
biological protection preventing irradiation of working personnel and of the environment;
thermal cooling ensuring an acceptable temperature level for a good preservation of the materials and the structures in which they are stored.
At present there are two main methods for the provisional storage of irradiated nuclear fuels, namely storage under water, which is the solution generally adopted on an industrial scale, and dry storage.
When stored under water, the irradiated fuels are immersed in the water of a tank. This water is permanently decontaminated by passage over purifying devices, such as filters or ion exchange resins. It is also permanently cooled by passag through a heat exchanger. Under these conditions the water ensures three main functions. Firstly it participates in the confinement by retaining the radioactive substances which have been able to escape from the fuels. It also provides a biological protection by stopping radiation, five to six meters of water providing an effective protection against most radiation. Finally, the water ensures the cooling of the waste by serving as a heat transfer agent which, during the passage through the exchanger, gives up the thermal energy emitted by the fuels.
This type of installation is used for receiving fuels resulting from the operation of reactors. This is the case with discharge or storage ponds, which fulfil a buffer function on entering fuel reprocessing plants. These installations produce high volumes of effluents, requiring significant capital expenditure and involving high operating costs.
In the other type of storage, i.e. dry storage, three main methods are used, namely storage in a flask, storage in a silo and storage in a cave.
In the case of flask storage, the fuels are placed in a cavity with metal walls surrounded by a system formed from different materials ensuring the biological protection. In this case, cooling takes place by conduction and radiation between the fuel and the metal wall serving as a confinement barrier and by convection and radiation between the confinement barrier and the environment.
In the case of silo storage, it is possible to use either buried silos, or surface silos. The fuel is placed in a confinement enclosure (generally constituted by a metal container with a welded cover) and is then placed in a generally concrete silo. The spacing between the silos is determined by local conditions. In the case of a buried silo, the heat transfer between the fuel and the confinement barrier takes place by conduction and radiation, whilst the heat transfer between the confinement barrier and the environment takes place by conduction. In the case of a surface silo, the heat transfer between the fuel and the confinement barrier takes place by conduction and radiation, whilst the heat transfer between the confinement barrier and the environment takes place by natural convection and by radiation.
In the case of cave storage, the waste is placed within vertical tubes, which are themselves distributed within a room closed or sealed by a biological protection barrier, e.g. a concrete wall. The lower part of the tubes are sealed and the upper part open, the irradiated fuel being introduced into the tubes by means of a transfer machine making it possible to pass the products to be stored through orifices appropriately arranged within the biological protection barrier. These orifices are located in the upper wall of the room and give access to the interior of the tubes from a handling area positioned above the room. The heat emitted by the fuel is evacuated by convection, generally by a circulation of air, which can be forced or natural.
In such an apparatus, cold air generally arrives in the lower part of the room, whilst hot air extraction takes place either in the upper part thereof, or in a side wall. Such an apparatus suffers from the deficiency that the air coming into contact with one of the tubes or shafts may already have been reheated on contact with another tube or shaft, which makes cooling relatively ineffective, or non-existent for certain tubes. This limits the storage capacity of certain rooms, or requires the presence of fans or other means ensuring a forced ventilation of the room, which makes such installations more complex and therefore more costly.